The present invention relates to a SQUID magnetometer.
The SQUID detector or SQUID (Superconducting QUantum Interference Device) is used for the measurement of weak magnetic fields. The SQUID has an output impedance of only about 1-5 W. Its operating temperature is typically 4.2 K, to which temperature it is cooled by suitable means. The operating temperature depends on the superconducting material used in the SQUID, therefore the operating temperature is as low as required for the operation of the superconductor. The noise at the output is very low; it is hardly higher than the thermal noise in the resistors used for the damping of the Josephson junctions. SQUID detectors are often used to measure low-frequency signals in the range of 0.1 Hz-10 kHz.
The problem with the type of detectors described above is that it is difficult to amplify the output signal without increasing the noise. The reason for this is that the range of voltage variation in the output signal is relatively narrow, about 10 .mu.V-100 .mu.V peak to peak. Moreover, the noise at the output of the SQUID roughly corresponds to the thermal noise of the resistors used for the damping of the Josephson junctions. If the noise temperature of an amplifier connected after a SQUID at 4.2 K temperature is below 10 K, the amplifier does not significantly increase the uncertainty of magnetic flux measurement.
No ordinary dc-connected amplifier operating at room temperature has sufficiently low output noise when connected directly to a SQUID. This is due to the low output imepdance of the SQUID and to the fact that it is often used to measure low-frequency signals in the range of 0.1 Hz-10 kHz. As FET amplifiers have a very low noise temperature in a frequency range of 1 kHz-100 kHz, flux modulation and a cryogenic transformer are used in connection with a dc SQUID detector to match the low impedance of the SQUID with the FET amplifier. With this arrangement, the uncertainty of the magnetic flux measurement is chiefly determined by the noise of the SQUID. Moreover, it can be proved both theoretically and experimentally that, as the SQUID noise is reduced, the magnetic flux-voltage conversion also increases so that the requirements regarding the signal processing electronics are not heightened.
However, problems ensue when several SQUID detectors are connected together to form multichannel devices. An example of such devices is the multichannel magnetometer. In recent times, these have been produced for the measurement of the weak magnetic fields of the brain and the heart. At present, the aim is to achieve magnetometers with 30-100 channels. If such a multichannel magnetometer is realized using SQUID detectors and the flux modulation technique, the costs of the signal processing electronics will be significant because each SQUID and channel requires a cryogenic transformer, preamplifier, modulator etc. The electronics of a multichannel magnetometer like this will be really complex and expensive.
The output signal of the SQUID can also be amplified by connecting the signal back to the SQUID's input via an extra coil and a resistor connected in series with it. The additional circuit thus formed causes positive feedback if the SQUID is current-biased.
Positive feedback amplifies the output signal, but it can also render the system instable. Instability can be controlled by replacing the resistor e.g. with a field-effect transistor, by means of which the resistance value can be easily adjusted.
If the SQUID is voltage-biased, an additional circuit can be used to cancel the noise in the output voltage of an operational amplifier. In this case, too, the electronics can be advantageously implemented using a variable resistor for the control of noise cancellation. The methods described above are reasonably simple, but generally they are not quite sufficient to reach the noise level of the SQUID.
Dc SQUID magnetometers are traditionally manufactured from two Josephson junctions with a damping resistor connected across them. If the Josephson junction is not resistively damped, it will have a hysteretic characteristic. Correspondingly, a dc SQUID composed of undamped junctions is hysteretic and cannot be used as a magnetometer. If the damping is insufficient, the dc SQUID will work but its noise may remain high. The thermal noise of the damping resistors limits the resolution of a dc SQUID, which is why the aim is to use a damping level as low as possible, yet so that the system remains stable. In practice, the junctions are often over-damped to ensure that the resonances associated with the dc SQUID connection circuits will not produce instability and increase the noise.